1. Field of the Invention
The invention relates to a communication method for carrying out Internet communication using a uni-directional communication line such as a satellite line and a communication apparatus to be applied to the communication method.
2. Description of the Related Art
Communication equipment (a transmitter) for transferring IP datagram flowing through a certain communication line to a uni-directional communication line such as a satellite line is called a feed. During transfer, the feed performs media conversion (encoding) for feeding data coming through a certain communication line through the uni-directional communication line. The feed is considered to be implemented as a router or a bridge. Some feeds functioning as a router implement UDLR (Uni-Directional Link Routing), which is a technique for using the uni-directional communication line as a bi-directional communication line in the form of a dummy (for UDLR, see Internet-Draft: draft-ietf-udlr-lltunnel-01.txt).
FIG. 1 shows the feed functioning as a router. In this case, a feed 1a selectively encodes the IP datagram flowing through a bi-directional communication line 3 in accordance with a destination address of the IP datagram and transfers the IP datagram to a uni-directional communication line 2. In this case, the feed 1a has two interfaces: an interface 4a to the uni-directional communication line 2 and an interface 5a to the bi-directional communication line 3. The two interfaces belong to different networks.
On the other hand, FIG. 2 shows the feed functioning as a bridge. In this case, a section for performing a function of the router is eliminated from the feed and this function is left to a general-purpose router 6, whereby a feed 1b can be used as an apparatus for encoding. In this case, the feed 1b has two interfaces: an interface 4b to the uni-directional communication line and an interface 5b to the router 6. The two interfaces belong to the same network.
Moreover, FIG. 3 shows another constitution of a bridge type feed supporting bi-directional communication as UDLR. A feed 101 has at least three network interfaces. It is now assumed that the interfaces are referred to as local I/F 102, UDL I/F 103 and tunnel I/F 104. The local I/F 102 is a bi-directional network interface. The network, which this I/F is connected to, is referred to as local network 105. The UDL I/F 103 is a uni-directional network interface for transmission only. The network, which this I/F is connected to, is referred to as UDL network 106. The tunnel I/F 104 is a bi-directional network interface. The network, which this I/F is connected to, is referred to as tunnel network 107. The local network 105 and the UDL network 106 have the same network address 108. The tunnel network 107 has a different network address 109 from the local network 105 and the UDL network 106.
A main function of the feed 101 is to receive all of packets flowing through the local network 105 (S201) and transfer these packets to the UDL network 106 through the UDL I/F 103 (S202), as shown in a flow chart of FIG. 4. This function is referred to as packet transfer function 110.
Another function of the feed 101 is as follows. In order to realize the bi-directional communication as UDLR, as shown in FIG. 6, this function includes to receive through the tunnel I/F 104 the packet called a GRE packet 111 (see RFC1701) and having another packet 112 encapsulated in the IP datagram (S401); to check whether or not the GRE packet 111 is fragmented (S402); to perform restructure process if the GRE packet 111 is fragmented (s403); to then extract the packet 112 encapsulated in the GRE packet 111 (S404); to make two copies of the extracted packet 112 (s405); and to send out one copy to the local network 105 through the local I/F 102 and send out the other copy to the UDL network 106 through the UDL I/F 103 (s406). This function is referred to as UDLR function 113. FIG. 5 shows the constitution in which the UDLR function 113 is executed.
When the feed is changed from the router to the bridge, the functions of the feed are reduced and thus implementation can be simplified. However, the UDLR, which is a technique for using the uni-directional communication line as the bi-directional communication line in the form of the dummy cannot be realized. This will be described with reference to FIG. 7. It is impossible to make transmission (9a or 9b) of the IP datagram from a router 7a at the receiving side on the uni-directional communication line to a router 8 at the transmitting side on the uni-directional communication line or to a router 7b at the other receiving side on the uni-directional communication line by virtually using the uni-directional communication line backward. The reason will be described below.
To transmit the IP datagram from the router 7a at the receiving side on the uni-directional communication line to the router 8 at the transmitting side on the uni-directional communication line or to the router 7b at the other receiving side on the uni-directional communication line by virtually using the uni-directional communication line backward, the IP datagram to be transmitted is encapsulated in the other IP datagram by using GRE (see RFC1701) and then the IP datagram is transmitted over the bi-directional communication line such as mainly a ground line. As shown in FIG. 8, IP datagram 16 to be transmitted is embedded in new IP datagram 15 as a data portion and then transmitted. However, a destination address 26 of the new IP datagram contains an IP address of the equipment (i.e., the feed) for realizing UDLR at the transmitting side.
In order that the feed receives the encapsulated IP datagram 15, the IP datagram having the IP address of the feed as the destination address 26 must be routed so that the IP datagram may be sent to the feed via the bi-directional communication line such as the ground line. When the feed functions as a router as shown in FIG. 9, the IP address of the interface 5a of the feed 1a to the bi-directional line is set to the destination address, whereby the GRE packet is routed without any problem and reaches to the feed through a route 10a. In this case, the transmission 9a of the IP datagram from the router 7a at the receiving side on the uni-directional communication line to the router 8 at the transmitting side on the uni-directional communication line by virtually using the uni-directional communication line backward is actually made via a route 11a of FIG. 10. The transmission 9b of the IP datagram from the router 7a at the receiving side on the uni-directional communication line to the router 7b at the other receiving side on the uni-directional communication line by virtually using the uni-directional communication line backward is actually made via a route 11b shown in FIG. 11.
However, when the feed functions as a bridge as shown in FIG. 12, the feed 1b is located close to the uni-directional communication line 2 compared to the router 7a at the receiving side on the uni-directional communication line. Thus, the interfaces (4b and 5b) of the feed 1b have the IP address belonging to the network address given to the uni-directional communication line 2. Thus, when the IP datagram is transmitted from the router 7a at the receiving side on the uni-directional communication line toward this IP address, the IP datagram is routed (10b) so that the IP datagram may be transmitted toward the uni-directional communication line 2, not toward the bi-directional communication line 3 such as the ground line. Since this route 10b is not realized due to properties of the uni-directional line, the IP datagram is not transmitted and consequently UDLR cannot be realized.
Next, a problem about connection between the feed and other equipment will be described. FIG. 13 shows one example of the form of the connection between the feed and other equipment. Besides the feed 101, a router 114 and a host 115 are connected to the local network 105. A receiver 117, a receiving apparatus having an interface 116 for reception only is connected to the UDL network 106. A router 118 is connected to the tunnel network 107. The router 118 may be the same router as the router 114. In this case, it is assumed that the local network 105 is connected to the tunnel network 107 through a different interface. In the prior art, as shown in FIG. 14, during the communication between the router 114 and the host 115 via the local network 105, a packet 119 to be transmitted is also transmitted to the UDL network 106 by the packet transfer function 110 of the feed 101. The packet transferred to the UDL network 106 is taken as a packet 120. However, the packet 120 is one which no equipment receives. A flow of such a packet through the UDL network 106 causes a waste of a band of the UDL network 106. Moreover, the equipment such as the receiver 117 connected to the UDL network 106 performs an operation such as rejection of this unnecessary packet 120 by filtering. Consequently, a resource such as CPU is wasted.
Moreover, as shown in FIG. 15, even when the packet 112 encapsulated in the GRE packet 111 received from the tunnel network 107 is addressed to the equipment connected to the local network 105, such as the router 114 and the host 115, the UDLR function 113 of the feed 101 causes the copy of the packet 112 to be transmitted to the UDL network 106. The copy of the packet 112 flowing through the UDL network 106 is taken as a packet 121. The packet 121 is one which no equipment receives, similarly to the packet 120. The flow of such a packet through the UDL network 106 causes the waste of the band of the UDL network 106. Moreover, the equipment such as the receiver 117 connected to the UDL network 106 performs the operation such as the rejection of this unnecessary packet 121 by filtering. As a result, the resource such as CPU is wasted.
It is a first object of the invention to allow the bi-directional communication by using the bridge type feed.
It is a second object of the invention to allow the feed to generate the least possible unnecessary packet such as the above-mentioned packets 120 and 121, to permit an effective use of the band of the UDL network, and to reduce a load on the equipment such as the receiver connected to the UDL network.